The leaching of sulphide materials using ferric sulphate to oxidise the sulphide mineral usually leads to rapid oxidation and thus dissolution of the required values. The surface potential of the mineral in such a case is usually in the range of 550 to 660 mV and the rate of leaching is henerally expected to increase with increasing potential. This is the case for such sulphide minerals as pyrite (FeS.sub.2); arseno-pyrite (FeAsS); chalcocite (Cu.sub.2 S); and sphalerite (ZnS).
However, in the case of chalcopyrite, leaching using ferric sulphate takes an inordinate length of time, presumably as a consequence of passivation of the exposed surfaces of the sulphide material. The exact nature of the mechanism whereby passivation occurs is not entirely clear and is presumed to be due to the formation of a passivating film on the chalcopyrite crystal surface which thus forms a diffusion barrier.
One attempt to overcome this difficulty has been the proposed use of activators or catalysts in the leaching system. One successful catalyst has been shown to be silver (see for example U.S. Pat. No. 3,856,913). However, as far as Applicant is aware, no commercial process has been developed to operate utilising such a catalyst presumably because of the high cost of silver and difficulties experienced in recovering it for recycle purposes. Other proposals have been the use of HgS; As.sub.2 S.sub.3 ; SnS; and CoS. None of these have proved to be commercially successful as far as the Applicant can determine.
Ahonen and Tuovinen, Hydrometallurgy, Vol. 24, No. 2, March 1990, 219-236 discloses the catalytic effects of silver in the microbiological leaching of finely ground chalcopyrite-containing ore materials in shake flasks. U.S. Pat. No. 4,571,387 relates to leaching copper from copper sulphide containing ore using sulphide oxidizing bacteria and a catalytic amount of silver.
It is, accordingly, the object of this invention to provide an effective process for leaching chalcopyrite without the use of any costly or deleterious activators or catalysts.